A color image forming apparatus having a plurality of image bearing members has advantages in producing copies faster than that having a single image bearing member and disadvantages that causes color shifts in an image. When respective toner images formed on the plurality of image bearing members are transferred onto a transferring member that is, for example, an intermediate transfer member to directly receive the toner images thereon or a recording medium conveyed by a sheet transferring member, it is difficult to overlay the respective toner images on accurate positions thereof, and a misalignment of the toner images may produce color shift on the overlaid toner image.
The color shift in image may be caused by various factors, and one of which is an image shift due to misalignment of components.
The image shift due to misalignment of components is a deviation of color formed at constant intervals on an image in the sub-scanning direction toward which a surface of an image bearing member rotates. The deviation of color is caused by a misalignment of components of an optical writing unit writing an electrostatic latent image on the surface of an image bearing member by emitting a laser beam toward the surface of the image bearing member. The deviation of color is also caused by a misalignment of a plurality of image bearing members. The image shift due to misalignment of components can be adjusted by controlling the timing of writing an electrostatic latent image by a laser beam. In the controlling method, a reference pattern toner image is formed on each image bearing member and transferred onto a transferring member. The reference pattern toner image transferred onto the transferring member is detected by a sensor or an image reading unit. Based on an output signal of the sensor, the amount of color shift in image is calculated and the timing of writing the electrostatic latent image is adjusted. However, when an optical writing unit in which a plurality of laser beams are emitted toward a polygon mirror is used, the image shift having the amount smaller than that a pitch in the sub-scanning direction of consecutive main scanning lines of a laser beam deflected by one mirror cannot be adjusted because of the structure of the optical writing unit itself.
In this connection, Japanese Patent Laid-open Publication No. 2004-198630 discloses a technique in which a plurality of image bearing members have respective drive motors serving as driving sources so that respective rotation speeds or linear velocities of the plurality of image bearing members can be separately controlled to adjust the image shift, according to the detection result of the reference pattern toner images formed on an intermediate transfer belt serving as a transferring member. The adjusting method used in the above-described image forming apparatus can reduce the image shift having the amount smaller than that of a pitch in the sub-scanning direction of the consecutive main scanning lines of the laser beam by changing the rotation speed of each image bearing member to make the deviation from respective predetermined positions of the reference pattern toner images formed on the intermediate transfer belt become small.
Another factor which may cause the color shift in image is rotation speed variations. The color shift in image, that is, the image shift due to rotation speed variations is a deviation of linear velocity in one rotation period of an image bearing member.
Regarding the image shift due to rotation speed variations, Japanese Patent Laid-open Publication No. 2000-298385 discloses a technique in which an adjustment of the phase of deviation in linear velocity in one rotation period of each image bearing member. The adjustment of the phase may be performed when respective rotations of a plurality of image bearing members are controlled to stop after the image forming operations are completed.
The method of adjusting the phase employs markings for aligning the phase on a predetermined position of a drive gear mounted on a shaft of each image bearing member according to the measurement result of deviation in linear velocity in each image bearing member previously measured. When the plurality of image bearing members are controlled to stop rotating, a sensor detects the above-described markings. According to the detection results by the sensor, the rotation of each image bearing member is controlled to stop so that the stop position of the rotation of each image bearing member may fall on predetermined positions between the image bearing members.
To reduce the image shift due to misalignment of components and the image shift due to rotation speed variations simultaneously, the above-described disclosed techniques were attempted to use in combination, which resulted in finding the following problem.
When an adjustment of the image shift due to misalignment of components is performed for a target image bearing member while the target image bearing member and the other image bearing members are driven to rotate at a speed identical to each other, the target image bearing member is fine adjusted, which creates a slight difference in rotation speed between the target image bearing member and the other image bearing members. When the target image bearing member and the other image bearing members are continuously rotated with the difference in rotation speed to perform a series of image forming operations, a difference in phase of linear velocities of the target image bearing member and the other image bearing members gradually become greater. For this reason, even through an adjustment of the image shift due to rotation speed variations is performed, the phase difference in linear velocity between these image bearing members may become clear enough to fine a visible color shift during the series of image forming operations, which may deteriorate the image shift due to rotation speed variation.